As of recent, there have been developed image forming apparatuses and image reading apparatus including optical apparatuses which have a lens array made up of multiple lenses. This configuration enables realization of reduction in apparatus size and costs, in comparison with configurations scanning a photosensitive member by a polygon mirror, configurations reading images using multiple lenses and mirrors, and so forth.
Japanese Patent Application Laid-Open No. 63-274915 (hereinafter referred to as “PTL 1”) discloses a lens array in which multiple lenses are arrayed in one direction (first direction). Each of the multiple lenses perform erecting same-size imaging of an object within a cross-section parallel to the first direction and optical axis direction (first cross-section), and perform inverted same-size imaging of an object within a cross-section perpendicular to the first direction (second cross-section). This configuration enables the lens power to be smaller within the second cross-section, as compared with an optical system performing erecting same-size imaging in the first cross-section. This is advantageous in realizing both resolution and light available efficiency.
Now, let us consider depth of field as being indicative of imaging capabilities of the lens array, in addition to resolution. Depth of field indicates a range on the optical axis over which a predetermined resolution can be obtained in front of and behind the image field position. Normally, a lens array having a great depth of field has lower light available efficiency, and a lens array having great light available efficiency has lower depth of field. Further, a lens array has to have resolution ensured within the first and second cross-sections, so consideration has to be given to common field of depth within both cross-sections.
However, the lens array disclosed in PTL 1 does not take into consideration the common depth of field within both the first and second cross-sections when receiving input of light rays from the light-emission points of an array light source. That is to say, the lens array described in PTL 1 is of a configuration where the depth of field in the first cross-section and the depth of field in the second cross-section are different. The common depth of field is determined by the smaller of the depths of field in both cross-sections, so the lens array according to PTL 1 has secured unnecessarily great depth of field in one cross-section. Accordingly, the lens array disclosed in PTL 1 is not an optimal configuration for realizing both resolution and light available efficiency, since light available efficiency is lost by the amount exceeding the common depth of field at one cross-section.
Also, the common depth of field of the lens array differs according to the position of each light-emitting point of the array light source, as well. Accordingly, difference in light-emitting point has to be taken into consideration to realize both resolution and light available efficiency, but there is no disclosure or suggestion of taking difference in light-emitting point with regard to the lens array described in PTL 1.